This disclosure relates to rotor angular position sensing for wound field synchronous machines (“WFSM”), and more particularly to a method of position sensorless operation of a wound field synchronous machine as a starter or motor using a carrier injection sensorless (“CIS”) approach with permanent magnet generator (“PMG”).
For aeronautical applications a WFSM is ideal for electromechanical power transfer systems. The WFSM may serve as both a starter/motor and generator when mechanically coupled to a variable speed prime mover, such as gas turbine engine. A typical WFSM includes a rotor which contains a main field rotor winding which is provided with electrical current from an excitation system. The excitation system consists of a exciter stator and exciter rotor. Both rotors are fixed to a shaft which is driven to rotate by a prime mover. The exciter rotor rotates adjacent an exciter stator, and the main field winding rotates adjacent a main stator.
Operation of a WFSM in generate mode constitutes a variable speed prime mover to drive the rotor shaft of the WFSM. The rotor shaft also includes or is coupled to a PMG rotor. During operation as a generator, the PMG is used to provide power to drive the excitation system and to power the controllers.
Operation of a WFSM in the motor mode constitutes a variable speed motor drive utilizing a solid-state power converter to process typically high potential direct current (“DC”) electric power to provide variable frequency AC power input to the WFSM. For operation of a WFSM as a variable speed motor drive it is necessary to know the rotational position of a WSFM main rotor to control the solid-state power converter to meet motor performance requirements.
Previous systems used position sensors (e.g., resolvers) to determine rotor position, which is an undesirable addition due to an increase in weight, size and complexity of the overall system. More recently other systems have used a back electromotive force (“EMF”) method to determine rotor position. However, the back-EMF method can not be used at standstill or low speeds due to insufficient back-EMF generated in the WFSM.
One method uses a carrier injection sensorless (“CIS”) algorithm to estimate the position of the rotor of a WFSM. A high frequency excitation signal with an electrical current or potential rotating waveform is applied to the WFSM directly at a high enough frequency that it sweeps around the stator faster than the rotor is turning, thus “viewing” the rotor from all angles. This “viewing” is possible by measuring the resulting rotating current or potential waveform, which contains information about the rotor due to rotor position dependent differences in the equivalent magnetic circuit of the WFSM. This technique works with any WFSM that has rotor saliencies that result in a change in impedance as seen at the stator windings to the high frequency excitation signal. This method requires that both the high frequency excitation and variable frequency AC power input be injected into the WFSM at the same time in order to meet motor performance requirements. The variable frequency AC power input will interfere in the operation of CIS and requires the high frequency excitation signal amplitude to be increased in order to meet motor mode requirements. In most cases this total input to the unit, variable frequency AC power input and high frequency and high frequency excitation signal, will equal or exceed the WFSM ratings, which may lead to reduced machine life and reliability problems.
One method uses CIS applied to the PMG to determine WFSM rotor position. However, this method requires that the number of poles of the PMG is a power of 2 submultiple (e.g., 1, 2, 4, etc.) of the number of poles of the WFSM main rotor, which can lead to an oversized PMG. This method also requires a PMG rotor to be precisely mechanically calibrated to the WFSM main rotor.